Born in Hadsund, Denmark, Christiansen was brought up by his Danish father and Spanish mother in Copenhagen. He joined local club Avedore IF at the age of nine, and remained there from 1982-1988, but after spending one year at Bronby IF
he joined Hvidovre IF in 1989. After moving to Spain in 1990, he had a spell with Real Madrid's youth team before almost immediately moving back to Denmark to Boldklubben. He scored six goals in a youth fixture against Kjobenhavns Boldklub and
then returned to Spain where he trained with FC Barcelona. In 1991 at the age of eighteen, he was signed on a four year contract under Manager Johan Cruyff and his outstanding team, which contained his Danish idol, Michael Laudrup. He soon
advanced to the Barcelona "B" team and after scoring twenty-eight goals in sixty games he gained international recognition in 1993, despite not having yet made the Barcelona first team. Due to foreign player rules he had agreed to take on Spanish
citizenship when he signed his four year contract and as soon as he was naturalised he was called up to the Spanish Under-twenty-one team in December 1992. He made his debut on 15th December 1992 in a 2-1 win over Germany at Bremer
Brucke, when he scored the Spanish equaliser in the forty-eighth minute and went on to play the full game. He went on to score three goals in eleven appearances at the Under-Twenty-one level. He went on to make his debut for the full Spanish team on
27th January 1993, when he came on as a half-time substitute for Kiko Narvaez in a 1-1 draw with Mexico at Las Palmas in the Canary Islands. A month later, he made his second appearance in a 5-0 win over Lithuania at Seville on 24th February 1993,
when he came on as a sixty-seventh minute substitute for Julio Salinas, with Spain already three goals to the good. He scored Spain's fourth goal in the eighty-sixth minute as they went on to win 5-0. As a striker, he had to compete with the likes of
Brazilian Romario, Bulgarian Hristo Stoichkov and Spaniard Txiki Begiristain for a place in the starting eleven and although he did not play in La Liga, he did make his club debut for FC Barcelona as an eighty-third minute substitute for Julio Salinas in
the UEFA Super Cup Final against Werder Bremen on 11th February 1992. On 13th February 1993, he was an unused substitute in the 1-1 home draw with Athletico Madrid in the Copa del Rey tie, but otherwise he never made the FC Barcelona team.
He went on loan to Sporting Gijon in February 1993, but injuries restricted him to four goals in ten games. He returned to his parent club but he picked up an injury in the pre-season and was then loaned to Osasuna, where he scored once in fourteen
appearances before returning to the Camp Nou at the end of the 1993-94 season. The following season saw him on loan with Racing Santander, where he scored once in fifteen appearances. The season had started well and he was recalled to the
National team squad, before injuries once more brought things to a halt. Due to contractual and Loan agreement problems, Barcelona had to sell Christiansen and he was signed first by Manchester City in October 1995, but he wished to remain in
Spain and was transferred to Real Oviedo in January 1996. He started well in his first season there, but struggled in his second and in November 1997 he was sold to Second Division Villarreal CF. He helped them to promotion in his first season there
but he could not sustain his form as the club were relegated in his second season and he was out of contract in 1999, after scoring six goals in forty-one appearances. He joined Terrassa FC in the Spanish lower League and scored five times in
seventeen games before joining Panionios FC in Greece, where he scored three times in twelve appearances, before returning to join the reigning champions, Herfolge BK, in his native Denmark in August 2000. He scored twice in a win over Bronby
and made four appearances for them before moving, in January 2001, to VfL Bochum in the German Bundesliga. Even though Bochum had been relegated at the end of the previous season, Christiansen scored seventeen goals his first season, to be
the club's leading goalscorer, as his side clinched promotion, and in his second season he again was the leading goalscorer with twenty-one goals and joint leading scorer in the Bundesliga. He had scored thity-eight goals in seventy-six appearances
and little wonder he attracted the attention of other clubs. He was signed by Hannover 96 to replace Fredi Bobic in June 2003. He scored nine goals in his first season before injury was again his downfall and curtailed his appearances as he sustained
knee and shin injuries. He scored twelve goals in fifty-five appearances before Hannover did not renew his contact at the end of the 2005-06 season and he retired soon afterwards. Seven years later Christiansen embarked on a coaching career, as
part of Luis Milla's coaching staff at Al Jazira in the United Emirates. He started there in February 2013 but soon left, in the October when the head coach was sacked. In April 2014, after being approached by former Barcelona B teammate, Xavier
Roca, who was the club's Director of Football, he was appointed head coach of AEK Larnaca FC of Cyprus. In his first season there, 2014-15, he guided AEK into second place in the Cyprus First Division behind APOEL FC, having lead the table for
almost the whole season, before being pipped at the post on the final matchday. Once more in 2015-16, he again piloted them into second place, once more behind APOEL, while in the UEFA Europa League they reached the Third Qualifying round
before being knocked out by FC Girondins de Bordeaux 4-0 on aggregate. In his two seasons at AEK they had won forty, drawn fourteen and lost fourteen of the sixty eight games played. After those two successful seasons at AEK, he joined APOEL,
the reigning Cypriot champions, on 21st May 2016, on a one year contract commencing June 2016. He was soon showing his skills as the club from Nicosia eliminated Norweigan side Rosenborg BK 4-1 on aggregate in the third qualifying round of
the UEFA Champions League. Unfortunately they could not progress further as they were knocked out of the Champions League in August 2016 after losing over two legs to FC Copenhagen in the play-off round. He did, however, manage to make
good progress in the UEFA Europa League, where he led APOEL to the last 16 of the UEFA Europa League for the first time in their history, after they topped a group which included European Giants such as Olympiacos F.C.. They then defeated
Spanish side Athletic Bilbao, 4-3 on aggregate over two legs, to progress into the last sixteen in the competition. It was a dream that was brought to an end when they narrowly failed to progress past R.S.C. Anderlecht at Constant Vanden Stock
Stadium, on 16th March 2017, but they left with their heads held high following a 0–1 defeat which coupled with their 0-1 home defeat, saw them lose 0-2 on aggregate. On the home front Christiansen won his first managerial title. APOEL were
crowned champions of Cyprus after losing just two games in the season and having the best defensive record in the league with twenty-seven clean sheets. They also were beaten finalists in the Cypriot Cup, losing 1-0, to a goal by Vinicius, against
Apollon Limassol on 24th May 2017. His record at APOEL was thirty-one wins, ten draws and eleven defeats in a total of fifty-two games. He amicably left APOEL to join Leeds united and the club wished him and his assistant coach Julio Bañuelos and
trainer Ivan Torres continued success in their careers. On 15 June 2017, Christiansen was announced as the new Head Coach of Leeds United, after being appointed by new Leeds Owner Andrea Radrizzani to replace the departed Garry Monk, with
the club announcing that they wanted to appoint someone who can help us create a winning culture at the club and unite everyone connected with Leeds United, from the players to the supporters. Four days later, it was revealed that he would be joined
by assistant Julio Bañuelos, fitness coach Iván Torres and goalkeeper coach Marcos Abad. He signed a two year contract with an option for a third. He had an almost perfect start at Leeds taking them nine games undefeated and to the leadership of
the EFL . Unfortunately this was followed by a succession of defeats as Leeds fell down the table. There was a rally over the Christmas period but it was soured by a series of player injury and suspensions and the ignomy of being eliminated from the
F.A. Cup by League Two side, Newport County, and then finally by Cardiff City inflicting a 1-4 home defeat on 3rd February 2018, which saw any pretence to promotion disappear. It saw the demise of Christiansen and he has not since found further
positions.